Large rotating shafts are used in many applications, such as turbines, fans, and pumps. The use in turbines often exposes them to environments and load cycling which can lead to cracking. Examples of use in the electric utility industry are high and intermediate pressure steam turbines, boiler feed pump turbines, and gas turbines.
High and intermediate pressure steam turbines are commonly used by modern electric utility companies for the purpose of driving generators to produce electric power. In order to operate safely and effectively, the components of the steam turbines, including the rotors, discs, blades, and stationary diaphragms are typically fabricated from heavy structural alloy steels of the chromium-molybdenum type. The high pressure steam turbines generally operate at temperatures higher than about 500.degree., however, certain components of the steam turbine are exposed to even higher operating temperatures. The hottest component of a steam turbine is typically the first stage of the rotor. During normal operation, sections of the rotor are typically exposed to temperatures higher than 850.degree. F., are often exposed to temperatures about 900.degree. F., and can be exposed to temperatures as high as 1050.degree. F.
The useful life of the various steel-fabricated steam turbine components depends heavily on the thermal stresses endured by the components. Because the rotors are typically exposed to much higher temperatures than the other components, the rotors tend to be susceptible to creep damage. In particular, the high temperature regions of the high pressure steam turbine rotors have increased susceptibility to cracking due to long-term thermal fatigue and creep from power plant load cycling. Similar damage, such as gouging, can occur from mechanical interference between the rotor and stationary components. In the past, the damaged rotor components have simply been replaced. However, replacement of the rotors requires very high capital costs and very long lead times for procurement.
An alternative approach which is currently used is to discard only the section of the rotor which is exposed to temperatures above 850.degree. F., including an appropriate portion surrounding the high temperature region, and replacing the discarded section with a new forging that can be welded to the remaining portion of the older rotor in a lower temperature region. In order for this approach to be successful, the replaced section of the rotor must be large enough to include both the high temperature region (i.e., above 850.degree. F.) and enough of the surrounding region that the weld can be made in an area where the operating temperature does not exceed about 850.degree. F. This conventional welding approach includes the use of a filler metal which cannot withstand prolonged exposure to temperatures higher than about 850.degree. F.
The filler metal used in the above-mentioned conventional welding approach is composed of about 0.12 weight percent C, 0.4-0.7 weight percent Mn, 0.4-0.7 weight percent Si, 0.025 weight percent P, 0.025 weight percent S, 0.20 weight percent Ni, 2.3-2.7 weight percent Cr, and 0.9-1.2 weight percent Mo. This conventional filler metal is commonly known as a 21/4 Cr-1Mo filler metal. The disadvantage of the conventional welding approach is that it represents an expensive repair program even if only minor cracking is observed in the high temperature region of the high pressure steam turbine rotor. Also, long lead times are often required for procurement of the replacement forgings.
Methods of repair in which a wide groove is machined in the rotor shaft to remove a crack and the groove is filled with weld metal are well known in the art. The wide groove may be semi-circular with a radius deep enough to fully remove the crack, or may be machined with straight walls and a large angle of greater than 24 degrees between opposing walls. The wide groove method requires a large amount of weld filler metal, increasing the welding time and chance of distortion. The large weld area also increases the chance of weld defects.
Various other welding filler metals are also known in the art for different high temperature applications. U.S. Pat. No. 4,994,647, issued to Tanaka et al., discloses welding deposit materials intended for use in high temperature applications (e.g., 482.degree. C. or 900.degree. F.). The disclosed uses are boilers, pressure vessels and chemical reactors. The reference teaches that 2.25-3% Cr--1% Mo steels are unsatisfactory for application to pressure vessels which are used under such high temperature and pressure conditions, such as required in a coal liquefaction plant. However, increased strength and increased resistance to hydrogen attack are imparted by adding V and Nb to the covered electrode.
U.S. Pat. No. 4,503,129, issued to Okuda et al., discloses a shielded metal arc welding electro for chromium-molybdenum low alloy steels. The reference states that chromium-molybdenum low alloy steels, including 2.25Cr--1Mo steel, are widely applied industrially as materials of high heat resistance in the fields of boilers of high temperature and pressure, petroleum industry, synthetic chemistry, and for uses requiring resistance to hydrogen of high temperature and pressure. The reference discloses a welding core wire and/or flux containing chromium and molybdenum, as well as carbon, manganese, silicon, aluminum, nitrogen and nickel. The reference further discloses that at least one element selected from vanadium, titanium, niobium and boron, can be added to the welding flux and/or the core wire to improve high temperature strength.
An article by Kim, et al. entitled "Weldability Studies In Cr--Mo--V Turbine Rotor Steel", Journal Of Materials Engineering, Vol. 10, No. 2, 1988, discusses a study undertaken to establish the weldability of a high pressure steam turbine rotor constructed using 1.0Cr--1.0Mo--0.25V steel. The reference discusses the performance of a post weld stress relief heat treatment to prevent cracking of the weld zone during subsequent exposure to elevated temperature service, and to restore its notch toughness. The reference discloses that a 2.25Cr--1Mo filler wire was used in the production of welds. After welding, and heat treatment, impact tests were performed on the weld specimens. The article concluded that postweld heat treatment at 1050.degree. F., caused a reduction in impact properties. On the other hand, postweld heat treatment at 1250.degree. F. caused some improvement in impact properties.
U.S. Pat. No. 4,897,519, issued to Clark et al., discloses a method for repairing worn surfaces of Cr--Mo--V steam turbine components which focuses on the use of a ferrous welding metal including about 4.00 to 19.0 weight percent chromium, 0.43 to 2.1 weight percent molybdenum, 0.09 to 0.5 weight percent vanadium, 0.03 to 0.20 weight percent niobium, 0.0 to 0.08 weight percent aluminum, 0.0 to 0.20 weight percent copper, 0.005 to 0.06 weight percent nitrogen, 0.04 to 0.22 weight percent carbon, 0.15 to 1.0 weight percent manganese, 0.15 to 1.0 weight percent silicon, 0.0 to 0.2 weight percent phosphorous, 0.0 to 0.016 weight percent sulfur, 0.0 to 0.8 weight percent nickel, and the balance iron. The alloy is welded to the worn surface of the turbine component using gas tungsten arc welding, plasma arc welding, electron beam welding, laser beam welding, or gas metal arc welding. The disclosed method is for repairing worn surfaces and no mention is made of repairing rotor shaft cracks.
U.S. Pat. No. 4,903,888, issued to Clark et al., is also directed toward a method for repairing worn surfaces of turbine rotors, including high pressure turbine rotors. A first layer of weld metal is deposited on a worn surface of a turbine component. Then, a second layer of weld metal is deposited over the first layer, using a higher application (weld heat input) temperature, for tempering at least part of the heat-affected zone created in the base metal by the depositing of the first layer.
U.S. Pat. No. 5,049,716, issued to Dunmire et al., discloses methods for providing erosion resistant surfaces to carbon steel turbine components. The surface is welded with a first weldment including steel having at least 12 weight percent chromium. The weldment is deposited on the carbon steel surface at a high rate of welding speed of about 24-52 inches per minute, in a first pass thickness of less than about 0.1 inches.
There is a demand among electric power utilities for a welding method which can be used to repair cracked high pressure steam turbine rotors without requiring extensive down time, capital expenditure, or replacement of rotor sections. It is important that the repair weld be able to meet or exceed the mechanical properties of the rotor base material, during and after long-term exposure to temperatures within the range of about 850.degree. F. to about 1050.degree. F. It is believed that such a welding method could extend the operating life of high pressure steam turbine rotors for an additional 10 to 20 years, resulting in savings of millions of dollars to electric utility companies.